Wireless communications such as the General Packet Radio Service/Universal Mobile Telecommunications System (“GPRS/UMTS”) and 1X Radio Transmission Technology/Code Division Multiple Access (“1XRTT/CDMA”) markets are evolving to enable multimedia services by converging on a common Session Initiation Protocol (“SIP”) based architecture such as those specified in the Third Generation Partnership Project (“3GPP”) IP Multimedia Subsystem (“IMS”) and 3GPP2 Multimedia Domain (“MMD”) standards initiatives. Although some differences exist between IMS and MMD, they are both based on a SIP signaling infrastructure with a Call Session Control Function (“CSCF”) node within the signaling and service control infrastructure as illustrated in FIG. 1. A Home Subscriber Server (“HSS”) 20 functions as a repository for user profiles, service subscription profiles, user identities and authentication data. A Serving CSCF (“S-CSCF”) 22 manages sessions and service control on behalf of a user based on user generated requests and user profile data stored in the HSS 20. An Interrogation CSCF (“I-CSCF”) 24 provides a defined entry point into the SIP signaling network, and enables SIP interconnection over operator network boundaries. A Proxy CSCF (“P-CSCF”) 26 functions as the user's SIP entry point into the SIP signaling network. The P-CSCF 26 is responsible for managing any access specific functions, such as data encryption, Quality of Service (“QoS”) control, and compression such that access details are not visible within the core CSCF network. The multimedia services are provided by application servers. The services are invoked from the S-CSCF 22 via an Intelligent Service Control (“ISC”) SIP-based interface 28.
Limited interworking with legacy service components is provided by gateways between the IMS protocols and the native legacy protocols. For example, an IMS Service Switching Function (“SSF”) 30 implements a virtual Intelligent Network (“IN”) SSF function, as well as an IP Multimedia Service Control (“ISC”)/SSF interworking function. This enables an IMS subscriber to invoke legacy Service Control Point (“SCP”) based services 32, such as Virtual Private Network (“VPN”) and Prepaid. A Media Gateway Control Function (“MGCF”) 34 and Media Gateway (“MGW”) 36 provide interworking between IMS based Voice over IP (“VoIP”) infrastructures 38 and legacy Public Switched Telephone Network (“PSTN”) networks 40. This enables an IMS based user to place a telephone call to a PSTN based subscriber.
One limitation of these SIP based architectures is that users must gain access using a specific multimedia device, e.g., IMS enabled device, over a suitable packet based access type, e.g., UMTS packet network, WLAN, etc. Consequently, non-IMS users are unable to invoke services that are integrated within the IMS.